perhaps its just personal preference? everyone has their own desires and reasons...

ABSOLUTELY!

I'm not looking to join anything-as I'm in a relationship with two.

If I were a single woman with no kids, I think joining a couple would be cool. I could keep my own place, have my personal space, not feel so "tied down", still have great times with my lovers without feeling guilty about them being "alone" whilst I was gone... Yep, I can see your point of view.